The present invention relates to a method and apparatus for testing the compression strength of a granule. More particularly, the present invention relates to a method and apparatus for determining the breakage mechanism involved with the compressive failure of a granule.
Many food products, detergents, pharmaceuticals and industrial intermediates are available as granular materials. Industry produces these granules by agglomeration processes from fine powders or slurries. Granulation conditions that are needed to obtain granules with acceptable mechanical properties are often set by trial and error, as a fundamental understanding of agglomeration processes and granule strength measurements has, before now, been limited. A number of particle strength tests are reported in literature, which often are only share in common the ability to generate and quantify particle breakage. Tests intended to assess granule breakage show a large spread in measured results or show very complicated particle-particle interaction. In both cases, interpretation of particle breakage results is difficult.
Particle breakage can be distinguished in attrition, fracture, abrasion and chipping. Small-scale damage due to normal forces is called attrition; large-scale catastrophic damage is called fracture. Small-scale incidental damage due to tangential forces results in polishing of the granule and is called abrasion. If impacts cause substantial tangential force on the granule and result in local damage to the particle surface, the breakage is called chipping.
U.S. Pat. No. 6,035,716 describes a repeated impact test method and apparatus that measures damage to granules under high velocity forces. This and other impact tests, however, do not predict damage to granules under static compression or low velocity forces. A need exists for a test method and apparatus to study damage and breakage of granules and particles under low velocity forces or static compression conditions.
Particle compression tests have been performed to obtain an insight in the mechanical quality of granules. In these tests, two parallel plates squeeze a granule after which the force and deformation of the particle is registered. Though the experiments are easy to conduct, interpretation of the obtained curves is in most cases complicated. Failure as a result of compression is a complicated phenomenon.
Compression experiments have been reported either with a constant deformation rate or with a constant loading rate. In a first method, two parallel plates compress a granule at a constant deformation rate. In a second method, two parallel plates compress granules with constant force increments. In the first method, compression with a constant deformation rate or compression with a constant indention velocity has to take place at an extremely low indention velocity. This in done in an attempt to monitor the breakage process, notwithstanding the fact that the measurements become very time consuming. A lot of attention has been paid in literature to the effect of compression speed on particle breakage force. Efforts have been made to deduce a material strength parameter from compression experiments (e.g. Yashima, 1979, Schulle, 1995). Regardless of such efforts, a method and apparatus for accurately monitoring the breakage process under compressive failure of a particle has still not been achieved, let alone for accurately monitoring the process.
The Zwick Texture Analyzer is a compression set-up based on this principle, as are compression test apparatus available from Instron. The Zwick device is equipped with with an accurate load cell to assess breakage strength of granules. It has a maximum load range of 50 Newtons and a resolution of 0.001 Newton. The device includes a blunt indentor that is moved by screw action from above a particle toward the particle on an anvil. The spatial resolution of the device is about 20 micrometers. Results with such a device show that only limited information about the breakage of the particle can be obtained. The spatial resolution information provided from such a device is insufficient for adequately studying breakage behavior. It was impossible to determine the difference between elastic and plastic loading. Furthermore, elastic energy is stored in the system.
In another method, compression with a constant loading rate or constant force increments does not allow monitoring of the breakage process itself for partly brittle particles. When the particle starts failing, the indentor moves further towards the granule while trying to increase the load on the particle. This leads to more damage to the granule, which then falls apart into fragments. Therefore no information is obtained about the breakage process of the particles.
In these and similar devices, elastic energy becomes stored in the system when a granule is tested and when the granule suddenly breaks, part of the stored energy is released and the indentor is accelerated toward the granule. Because the granule has no effective means of draining this energy, it is likely to collapse.
The use of a compression apparatus including a piezoelectric device has been tried with a constant indentation speed mode and provides good spatial resolution. However, at the point of breakage no results are obtained because the granule tested is smashed.
All references mentioned herein are incorporated herein in their entireties by reference.
A need exists for an apparatus and method for accurately and more quickly monitoring the breakage process of a particle under compression testing conditions and for determining the breakage mechanism of the particle.
A need further exists for an apparatus and method for accurately and more quickly monitoring repeated compression testing on a particle and for determining the breakage mechanism of the particle.
The present invention provides a method and apparatus for the accurate monitoring of a granule during compression testing and for the determination of the breakage mechanism that causes particle failure. Although the invention will be described in connection with the testing of granules, particles, or both, it is to be understood that the invention is applicable for the testing of other objects as well, such as particles, agglomerates, molded objects, and the like.
According to the present invention, a granule is compressed with a double spring compression device that enables the study of the process of granule breakage in more detail. This is achieved by a combination of a spring in parallel and a spring in series with the granule. With the apparatus of the present invention, sudden deformation of the granule results in fast relaxation of the granule during breakage and allows very careful granule breakage.
The apparatus according to an embodiment of the present invention includes a bottom plate, a platform, an indentor, top and bottom biasing members, and a measuring or recording device. The bottom plate has a top surface and a bottom surface. The platform has a top surface and an opposite bottom surface and the bottom surface faces the bottom plate. The platform top surface includes a biasing portion, and a contact portion for contacting a granule to be tested. The bottom biasing member is positioned between the bottom plate and the platform and is preferably in contact with both the top surface of the bottom plate and the bottom surface of the platform. The bottom biasing member biases or forces the platform away from the bottom plate. The indentor has an indentor bottom surface facing the platform and a contact surface extending below the indentor bottom surface in a direction toward the platform. The indentor contact surface faces the platform and contacts a granule to be tested that is positioned on the platform. The top biasing member is positioned between the top surface of the platform and the indentor bottom surface and is in contact with both the indentor bottom surface and the biasing portion of the platform top surface. The top biasing member biases or forces the indentor away from the platform. The measuring or recording device measures the force exerted on a granule to be tested under force exertion conditions.
The method of the present invention involves using an apparatus of the present invention to study the compression characteristics and breakage mechanisms of a granule or particle. The method includes: providing a compression testing apparatus as described above; positioning a granule to be tested on the platform in contact with the contact portion of the platform and between the contact portion of the platform and the contact surface of the indentor; exerting a force on the indentor in a direction toward the platform sufficient to move the contact surface of the indentor into contact with the granule and to exert a compressive force on the granule; and measuring the compressive force exerted on the granule. The method can further involve recording the measured forces and determining a breakage mechanism by analyzing the measured forces.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.